Microstructure and Mechanical Properties of Aluminum Alloy/Stainless Steel Dissimilar Ring Joint Welded by Inertia Friction Welding

In recent years, studying the weldability of a dissimilar metal hybrid structure, with the potential to make full use of their unique benefits, has been a research hotspot. In this article, inertia friction welding was utilized to join Φ130 forged ring of 2219 aluminum alloy with 304 stainless steel. Optical observation (OM), electron back scattering diffraction (EBSD), and scanning electron microscopy (SEM) were utilized to examine the joint microstructure in depth. Depending on the research, a significant thermal–mechanical coupling effect occurs during welding, resulting in inadequate recrystallization on aluminum-side thermo-mechanically affected zone (TMAZ) and forming zonal features. The crystal orientation and grain size of each TMAZ region reflect distinct differences. On the joint faying surface, the growth of intermetallic compounds (IMCs) is inhibited by a fast cooling rate and metallurgical bonding characteristics were found depending on the discontinuous distribution of IMCs. The average joint tensile strength can reach 161.3 MPa achieving 92.2% of 2219-O; fracture occurs on aluminum-side base metal presenting ductile fracture characteristics.

Observation of microstructure evolution during inertia friction welding using in-situ synchrotron X-ray diffraction

The widespread use and development of inertia friction welding is currently restricted by an incomplete understanding of the deformation mechanisms and microstructure evolution during the process. Understanding phase transformations and lattice strains during inertia friction welding is essential for the development of robust numerical models capable of determining optimized process parameters and reducing the requirement for costly experimental trials. A unique compact rig has been designed and used in-situ with a high-speed synchrotron X-ray diffraction instrument to investigate the microstructure evolution during inertia friction welding of a high-carbon steel (BS1407). At the contact interface, the transformation from ferrite to austenite was captured in great detail, allowing for analysis of the phase fractions during the process. Measurement of the thermal response of the weld reveals that the transformation to austenite occurs 230 °C below the equilibrium start temperature of 725 °C. It is concluded that the localization of large strains around the contact interface produced as the specimens deform assists this non-equilibrium phase transformation.

Advanced characterisation of microstructural evolution and liquation mechanisms in RR1000 employing a novel semi-solid testing facility

Inertia friction welding is a joining technique that is used in many industries as it can create high quality welds with narrow heat affected zones. The nickel superalloy RR1000 is routinely joined via inertia friction welding, during the manufacture of compressor components for gas turbine engines, at Rolls-Royce Plc. The conditions experienced at the weld interface of an inertia friction weld are extreme, with a combination of rapid heating rates to high temperatures and severe strain rates. These conditions are such that liquation of RR1000 is to be expected. The liquation mechanisms of two variants of RR1000 have been investigated, building on previous research, to further understand the dynamic evolution of microstructure and mechanical properties during inertia friction welding of RR1000. This investigation was accomplished by the design and commissioning of a novel semi-solid testing facility. The facility underwent numerous modifications to allow representative replication of the conditions experienced during inertia friction welding. To investigate the liquation mechanisms occurring in RR1000, fine grain and coarse grain compression specimens were heated to temperatures between 900°C and 1200°C at heating rates between 1°Cs-1 and 25°Cs-1. The fine grain and coarse grain variants were found to liquate via two mechanisms. The fine grain primarily experienced constitutional liquation of the primary γ’ precipitates, while incipient melting of the γ phase was experienced by the coarse grain variant. The liquid propagation rate in both variants was characterised. The knowledge and understanding gained via the use of this facility was then applied to analyse the microstructures from a series of interrupted RR1000 inertia friction welds. Inspection of these interrupted welds revealed evidence of liquation like that observed in the specimens tested in the semi-solid testing facility. This research has given an insight into the role of liquation during inertia friction welding of RR1000.